Digital communication receivers sample an incoming waveform and then reliably detect the sampled data. Typically, a receiver includes a Clock and Data Recovery (CDR) system to recover the clock and data from an incoming data stream. The CDR system generates a clock signal having the same frequency and phase as the incoming signal, which is then used to sample the received signal and detect the transmitted data.
The quality of the received signal is often impaired by intersymbol interference (ISI), crosstalk, echo, and other noise. In addition, impairments in the receiver itself may further degrade the quality of the received signal. The received signal can be viewed as a well-known “data eye,” which is a superposition of a number of impaired individual signals with varying frequency components, for example, due to ISI and other noise. As the various impairments increase, the quality of the data eye associated with the received signal is impaired. An important characteristic in the recovery process is the height and width of the data eye. Generally, as the number of frequency components associated with the received signal increases, the data eye becomes less open. The accuracy of the data detection significantly degrades as the data eye becomes less open.
“Eye margining” is a technique by which the height and width, or margins, of a data eye can be measured. The eye margin of a receiver can be evaluated following the manufacturing process, or prior to deployment in a given application, to determine if the receiver satisfies one or more predefined margin criteria. If the receiver does not satisfy the one or more predefined margin criteria, the device can be rejected or one or more device parameters can be modified and the margin criteria can be reevaluated. Eye margining is often performed using a classical jitter tolerance technique and eye histogram techniques.
A need therefore exists for automated methods and apparatus for evaluating the eye margin of a communications device. A further need exists for methods and apparatus for evaluating the eye margin of a communications device using a data eye monitor.